The invention relates to a process for the adsorptive separation of a xylene, preferably para-xylene, from a feed stream containing an admixture of C8 aromatic hydrocarbons. More specifically the invention relates to improving the performance of a simulated moving bed liquid-phase adsorptive separation process for the recovery of para-xylene from other C8 aromatic hydrocarbons.
Simulated moving bed (SMB) technology is used commercially in a number of large scale petrochemical separations. The general technique employed in the performance of a simulated moving bed adsorptive separation is well described in the open literature. For instance a general description directed to the recovery of para-xylene was presented at page 70 of the September 1970 edition of Chemical Engineering Progress (Vol. 66, No 9). A generalized description of the process with an emphasis on mathematical modeling was given at the International Conference on xe2x80x9cFundamentals of Adsorptionxe2x80x9d, Schloss Elmau, Upper Bavaria, Germany on May 6-11, 1983, by D. B. Broughton and S. A. Gembicki. Numerous other available references describe many of the mechanical parts of a simulated moving bed system, including rotary valves for distributing various liquid flows, the internals of the adsorbent chambers and control systems.
U.S. Pat. No. 3,686,342 issued to R. W. Neuzil describes the separation of para-xylene from mixed xylenes using simulated countercurrent adsorption employing a zeolitic adsorbent and para diethylbenzene as the desorbent. This combination is a good representation of a commercial operation for this particular separation.
U.S. Pat. No. 3,510,423 to R. W. Neuzil et al. provides a depiction of the customary manner of handling the raffinate and extract streams removed from an SMB process, with the desorbent being recovered from each of these streams by fractional distillation, combined and recycled to the adsorption zone. U.S. Pat. No. 4,006,197 to H. J. Bieser extends this teaching on desorbent recycling to three component desorbent mixtures. U.S. Pat. No. 4,036,745 describes the use of dual desorbents with a single adsorption zone to provide a higher purity paraffin extract.
U.S. Pat. No. 5,948,950 issued to G. Hotier et al describes a process for separating para-xylene from a C8 feed in a simulated moving bed process using a zeolitic adsorbent and a desorbent, which can be toluene. The patent stresses the importance of proper hydration of the zeolite to good separation performance, with the hydration level being maintained by water injection into one of the process streams circulating through the adsorbent. The desorbent to feed ratio (S/F) disclosed in this reference varies from 0.6 to 2.5. The reference describes the use of several molecular sieve based adsorbents including barium and potassium exchanged X and Y zeolites. The performance of the process is measured in terms of a performance index designated IP. This reference, like the others cited above, is silent as to the importance of desorbent purity to the performance of the overall process.
The invention is an adsorptive separation process for the recovery of para xylene from a mixture of C8 aromatic hydrocarbons. The process is operated at conditions, including high desorbent purity, providing improved overall performance at the same volumetric ratio of desorbent to feed (D/F). The invention allows the D/F ratio to be more widely varied depending on feed composition, target performance levels and desired adsorption zone operating temperature.
The invention may be characterized as a simulated moving bed adsorptive separation process for the separation of para-xylene from a feed mixture comprising at least one other C8 aromatic in which the feed mixture is contacted with a zeolitic molecular sieve, which sieve has been exchanged with at least barium, at adsorption promoting conditions including a temperature ranging from 210-300xc2x0 F., para xylene is selectively adsorbed on the molecular sieve and subsequently removed using a desorbent stream comprising toluene, characterized by maintaining a performance index above 90 at an A/F ratio of from about 0.5 to about 0.7 by operating with a toluene purity in the adsorbent above 98 vol. % allowing operation at an L3/A ratio of about 1.6 to about 2.3, where
A=rate of simulated circulation of selective pore volume through the process,
F=volumetric feed rate of the feed mixture,
L3=liquid flow rate through zone 3 and
IP=square root of (yield % times purity %) of para xylene in the extract product stream of the process.